Aims.We performed a deep wide field optical survey of the young
(~ Myr) open cluster Blanco 1 to study its low mass population well
down into the brown dwarf regime and estimate its mass function over
the whole cluster mass range.

Methods.The survey covers 2.3 square degrees in the I and z-bands down
to with the CFH12K camera. Considering two
different cluster ages (100 and 150 Myr), we selected cluster member
candidates on the basis of their location in the () CMD
relative to the isochrones, and estimated the contamination by
foreground late-type field dwarfs using statistical arguments,
infrared photometry and low-resolution optical spectroscopy.

Results.We find that our survey should contain about 57% of the cluster
members in the mass range, including 30–40 brown
dwarfs. The candidate's radial distribution presents evidence that
mass segregation has already occured in the cluster. We took it into
account to estimate the cluster mass function across the
stellar/substellar boundary. We find that, between and
, the cluster mass distribution does not depend much on
its exact age, and is well represented by a single power-law, with
an index . Over the whole mass domain, from
0.03 to 3 , the mass function is better fitted by a
log-normal function with and
.

Conclusions.Comparison between the Blanco 1 mass function, other young open
clusters' MF, and the galactic disc MF suggests that the IMF, from
the substellar domain to the higher mass part, does not depend much
on initial conditions. We discuss the implications of this result on
theories developed to date to explain the origin of the mass
distribution.

Based on observations obtained at
the Canada-France-Hawaii Telescope (CFHT) which is operated by the
National Research Council of Canada, the Institut National des
Sciences de l'Univers of the Centre National de la Recherche
Scientifique of France, and the University of Hawaii. Based on
observations collected at the European Southern Observatory, Chile
(ESO programmes 68.C-0233 and 71.C-0446). Some of the data
presented herein were obtained at the W.M. Keck Observatory, which
is operated as a scientific partnership among the California
Institute of Technology, the University of California and the
National Aeronautics and Space Administration.

**

The list of low mass star candidate members and Table 3 are available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/471/499

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